Automatic focus means to control a distance from a voice coil motor (Voice Coil Motor, VCM) to an image sensor by using an auto-focus algorithm, so that a subject of a to-be-photographed object is clearest.
Common auto-focus algorithms may be classified into two types: an active focus algorithm, that is, a ranging method, and a passive focus algorithm, that is, a hill climbing method.
In the ranging method, a distance from a to-be-photographed subject to a lens, that is, an object distance, is measured by using a method such as infrared ranging, ultrasonic ranging, or binocular stereoscopic vision, a VCM position corresponding to the object distance is calculated, and the VCM position is adjusted to obtain an in-focus image. The ranging method is characterized by a relatively fast focusing speed, but an additional ranging module is required. Therefore, costs are high, and a relatively high precision requirement is imposed on the ranging module. As shown in FIG. 1, a VCM position corresponding to a measured object distance is calculated according to the object distance, and an in-focus location in the case of an optimal contrast (contrast) is directly determined, to obtain an in-focus image.
In the hill climbing method, a VCM position is changed to obtain images corresponding to different VCM positions, and a contrast of the images and a contrast changing trend are analyzed, to find a VCM position in the case of an optimal contrast, and obtain an in-focus image. The hill climbing method is characterized by a relatively low focusing speed, but relatively low costs and relatively high focusing precision. As shown in FIG. 2, a VCM position corresponding to an optimal contrast is determined by means of analysis to complete focus, to obtain an in-focus image.
However, when the hill climbing method is used for focusing, a travelling range of all VCMs is usually not searched for, but instead, the first found peak value in a contrast curve is determined as an in-focus location, to quickly obtain an in-focus image. However, in some scenarios, such as a macro scenario, some objects in a focus window are quite close to a lens, and some objects are quite far away from the lens. In this case, the contrast curve is a curve with two peaks, and consequently, a focus result may be a background part in image content, and an ideal in-focus image cannot be obtained.
For example, as shown in FIG. 3, in a macro scenario, there is a double-peak contrast curve shown in the figure. Due to different VCM positions, the contrast curve has a peak value at a closer location of a flower that is corresponding to a black focus window, and has a second peak value at a location of a remote hill that is corresponding to a white focus window.
For another example, in focusing performed by using the hill climbing method, a central area in a previewed picture is usually selected as a focus window, but sometimes the central area is not an area that a user expects to focus. Therefore, focusing performed by using a central focus window may lead to out-of-focus, and an ideal in-focus image cannot be obtained.
In addition, for focusing of a flat area, for example, in a flat area scenario such as sky, there is no object with obvious rich texture in a focus window, and there is no obvious contrast curve peak value in a motor travelling range. Therefore, it may be difficult to find an in-focus location, even out-of-focus is caused, and consequently, an ideal in-focus image cannot be obtained. Likewise, in a face focus scenario, a face area is not an area of extremely rich texture either. If focusing is performed only by using the face area, an in-focus location that is eventually determined may be not accurate enough either because a peak in a contrast curve is not sharp enough. In addition, image noise or movement of a photographed person or a camera device in a focus process greatly affects determining of a peak value in the contrast curve. For example, as shown in FIG. 4, image content is a sky scenario. A contrast curve corresponding to this image is quite flat, and it is difficult to determine an accurate peak value. Consequently, an in-focus location that is eventually determined may be incorrect, and an ideal in-focus image cannot be obtained.
In conclusion, in a process of automatic focus performed by using the hill climbing method, in different photographing scenarios, because a focus window search range is limited, or texture of image content in a focus window is not rich enough, it may be difficult to find an in-focus location, even out-of-focus is caused, and consequently, an ideal in-focus image cannot be obtained.